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Both Cincinnati and Chicago excelled in the slaughterhouse business, and both would be called “Porkopolis.” Cincinnati has the nickname “Porkopolis” as early as 1843, and Chicago claimed the title in 1862. The inter-city rivalry would result in the “Windy City” nickname a few years later.

(Oxford English Dictionary)
-opolis, comb. form
Forming names or nicknames of cities or towns, in which the first element of the noun represents a major export, trade, etc., for which the city or town has become well known (e.g. COTTONOPOLIS n.), or occas. some other characteristic (e.g. PARASITOPOLIS n.).
1846 Gettysburg (Pa.) Republican Compiler 22 June 1/3 The gentleman..has made himself somewhat famous..in furnishing some important..facts in relation to the social condition of the inhabitants of ‘Porkopolis’.

20 May 1843, Brother Jonathan, pg. 89:
J. M. FIELD and TOM PLACIDE appear to be great favorites in Porkopolis.

26 December 1843, Cleveland Herald, “The Cincinnati Forgery,” pg.2, col. 4:
The affair has created a great sensation in “Porkopolis.”

3 January 1846, Massachusetts Ploughman and New England Journal of Agriculture, pg. 1:
CINCINNATI PORK BUSINESS.
A correspondent of the Traveller, writing from CIncinnati, Ohio, under the date of November 29th, gives the following description of the way they do business in the pork line in that great Pork-opolis of America:...

10 April 1862, Chicago Tribune, pg. 1:
The Beef and Pork-packing season in Chicago for 1861-62 is now closed, and the result exceeds the highest anticipations of the trade. We have long held the position of beingthe greatest Beef-packing market in the world; but even the most sanguine of our citizens did not dream of taking from Cincinnati the title of Porkopolis, at least for ten years to come. But the facts show that while Cincinnati this last season has packed 483,000 hogs, there have been packed in Chicago by regularly established packers, no less than 514,118 hogs, besides 55,212 beeves.

24 March 1863, Chicago Tribune, pg. 1:
In former years Cincinnati ruled supreme as a pork packing point, and for a long time has borne the sobriquet of Porkopolis, but the season of 1861-62 demonstrated that she was no longer entitled to this honorable distinction, as this city packed some 40,000 more than she did.

28 March 1863, New York Times, pg. 4:
THE NEW “PORKOPOLIS.”—Cincinnati has lost her chief glory, and must henceforth resign with that the soubriquet of which she has long boasted. Westward the hog wends his way, and over Chicago, the City of the Lake, his star now rises and sets.